The Mozilla Foundation has just released version 52 of the Firefox browser. One of the significant changes is dropping support for NPAPI.
We have prepared for this since StoreFront 3.8 by activating the Citrix Receiver Launcher for Firefox 53 and later. This is because the information available at the time suggested that NPAPI support would be removed in Firefox 53.
As it happened earlier than expected, you have to edit the web.config file under the Receiver for Web site (typically C:\inetpub\wwwroot\Citrix\StoreWeb) to activate the Citrix Receiver Launcher for Firefox 52.
Open web.config using your preferred text editor and locate the line highlighted in the screen shot:
This is a regular expression specifying that the Citrix Receiver Launcher is used for Firefox 53 and later. Change the highlighted bit to (Firefox/((5[2-9]|[0-9])|\d\d\d)) to make Receiver for Web use the Citrix Receiver Launcher for Firefox 52 onward.